In reality, a fragmentation grenade is essentially a "Typhoon-like" weapon which sprays thousands of metal fragments or steel balls in all directions when it explodes.

In Deus Ex, the "fragmentation" grenade is more like a high-explosive grenade (a real life concussion grenade) which causes trauma by using shockwaves.

Fragmentation grenades are noisy, but effective against all enemies (organic or not). Against bosses, they do large chunks of damage. The trick is to get the grenade to detonate when you want them to, as the frag grenade uses a timed fuse (and not proximity or impact fuses).